The object of this research is to investigate the mechanism for the differential synthesis of HbA and HbF in human fetal and adult erythroid cells. In vitro studies are being performed of hemoglobin synthesis b erythroid colonies grown in plasma clot cultures from human fetal liver tissue and umbilical cord bloods. Beta/gamma synthesis ratios are measured in the clots utilizing 3H-leucine incorporation and compared to globin chain synthesis in the tissue from which the colony precursors were obtained. The influence of added hemin, estrogens, androgens, adrenal cortical hormones and other compounds will be assayed.